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DTH Drilling Tools in Complex Formations
Aug 08, 2018

The so-called complex stratum refers to the development of joints, lithologies and fissures of the strata due to the influence of genesis, tectonic movement and weathering and groundwater; soft and hard interbed, fracture, cementation, stability, strength, etc., or It is inflated with water. Construction in such formations, once drilled through the borehole, its original relatively stable or equilibrium state is destroyed, causing the borehole wall to lose its constraint and cause instability. 

Common phenomena are wall collapse, blockage, loss, water inrush, diameter reduction, and ultra-diameter. When the borehole is bored in the upper stratum, the wall is unstable and the retaining wall is difficult; when the geological core is drilled, the core is broken, washed and dissolved, and the rock core is taken into a difficult problem; the gravel layer contains the lone stone and the boulders. The weathered layer contains weathered balls, weathered nuclei, or uneven rock soft and hard. It is not only inefficient when drilling, but also difficult to form holes according to design trajectory, that is, anti-slanting or increasing drilling speed becomes a prominent problem.

Practice has shown that air downhole impactor drilling has the following advantages:

1) High drilling efficiency. It is 2 to 5 times more efficient than the commonly used hydraulic impactor; it is 3 to 10 times higher than the diamond rotary drilling. The reason for improving efficiency is that the single impact energy is large, and there is no liquid column pressure; the slag discharge speed is high, the bottom of the hole is clean, and there is no secondary crushing; the condition of crushing rock at the bottom of the hole is improved;

2) The cylindrical cemented carbide drill bit used in the hard rock is not only good for rock breaking, but also has higher life than diamond bit;

3) The drilling speed is low, the centrifugal force is small, the impact chance of the drilling tool on the hole wall is small, and the drilling method is high-frequency impact on the bottom of the hole, which reduces the influence of the hole inclination on the fractured or inclined formation. Thereby, the verticality of the borehole can be increased, and the rock collapse of the bore wall can also be reduced;

4) Drilling pressure and torque required for drilling are much smaller than that of rotary drilling, which can reduce the weight and capacity of the supporting drilling equipment, and create favorable construction conditions for the slope anti-sliding reinforcement drilling operation on the high-altitude shelf;

5) Drilling adopts liquid-free circulating dry operation, which does not pollute the environment and prevents the large rock mass from sliding on the slope.

Its disadvantages:

1) Since the dynamic medium and the circulating medium are compressed air, the balance of the support of the hole wall is poor, and the retaining wall is very unfavorable;

2) The mud cannot be used as the circulating medium for drilling, and it is even more difficult to use the wall treatment agent and the plugging material;

3) The broken rock is a dynamic load impact. The drill bit impacts and vibrates on the rock, which makes it easier to cause vibration and fracture of the hole wall or core, which brings extremely unfavorable conditions to the retaining wall and the core work;

In view of the above shortcomings of the DTH impactor, the author has studied the structure of the DTH impactor, the matching of the drill, the type of the drill bit, and the drilling process for many years, so that it can be adapted to drill in the complex formation. Several technical measures have been introduced.

Casing isolation and hole drilling

1. Asynchronous heel drilling method (conventional casing method)

First, use a down-the-hole impactor to break the rock, and then the lower casing is isolated, and then use a small first-level drilling tool to continue drilling in the casing. This is a common method of drilling holes. During the construction, the drilling speed should be increased as much as possible, and the crushing belt should be quickly passed. The purpose is to shorten the action time of the external factors (flushing liquid soaking), so that the wall of the hole can be kept stable for a certain period of time, so that part of the time can be won, and then the casing is isolated. Hole guard.

When the author conducted engineering surveys in a certain place in Guangdong in 1995, he used a down-hole impactor to drill. The survey port area is built by the reclamation of Lushan Mountain. The upper part of the stratum is filled with earth and rock, the middle part is residual silt soil, and the lower part is continental sedimentary rock (Fig. 1). For the port area, the stratum is mainly filled with rockfill. The rock is broken and hard. The GC-110 type DTH impactor is equipped with a 130 mm drill bit. The naked eye is drilled into the same diameter. After passing through the muddy soil layer, the next 108 mm set. After the tube is cleaned, the original soil is taken and the standard test is carried out.


Figure 1 Yantian Port Engineering Survey Geology Profile

1—granite; 2—hard riprap with muddy sand; 3—silt soil and silt;

4—Asian sand, sub-clay; 5—sea water

Problems and countermeasures in drilling: (1) Collapse and falling off often occur during drilling. Therefore, the hole is quickly formed, and the hole is avoided as much as possible. (2) When the drilling depth reaches below the sea surface, the backfill, sand and seawater are mixed to form a mud pack, and the paste is in the annular gap between the drill pipe and the drill hole. The working air pressure is increased, the exhaust cycle is weakened or stopped, and the working efficiency of the down-the-hole impactor is reduced or even not working, and the drill bit is seriously worn, so that the footage is slow and the footage is not advanced. After these phenomena appear in the drilling, a certain amount of foaming agent (actually replaced by washing powder) is poured into the drill pipe to dilute the mud bag, and the malfunction can be eliminated and the normal work can be resumed.

The construction shows that the aging time of the submerged hole impactor in the accumulation layer can reach 15.6 m/h, up to 21 m/h; the life of the submerged hole impactor can reach 80 m/only. Although the density of the riprap deposits in the port area is poor, and there are holes in the individual holes in the seawater, the air-drilling hole impactor is a low-density medium, which has a small pressure on the hole wall and avoids the treatment of the leak. Time and material consumption are required; although the riprap stack is uneven in hardness and hardness, all boreholes are not obscured by the hole; although the survey hole is broken and collapsed, it is extremely unstable, but due to the drilling efficiency of the downhole impactor High, the naked eye time is short, can all go to the end, only need to make a hole in the next layer, and then sample or do the standard test, which simplifies the drilling structure.

From January to March 1997, the author used a down-the-hole impactor to drill blastholes in the underwater complex of the Songhua River in Jilin Province. The purpose of the project was to exploit the gravel and bedrock at the bottom of the Songhua River to 5.5 m to form a river water flow. Go to the shore storage tank to divert water buns and bury the water pipeline foundation. The total length of the project is 192 m, the width is 82 m to 12 m, and the total number of holes is 845. Due to the undulating and uneven surface of the river, the water depth varies greatly. The top layer has 25% to 30% sand gravel layer from top to bottom, with an average particle size of 0.04 m to 0.3 m and a maximum of 1 m; 2 weathered basalt layer with a thickness of 0.5 m to 0.8 m, crack development and fracture ; 3 more complete bedrock, dark green cryptocrystalline, drillability up to 8; 4 bedrock with 0.8 m ~ 1.2 m cracks, fractured zone containing pores; below the fracture zone is a hard basalt formation (Figure 2 ).


Figure 2 GC-100 type DTH impactor construction equipment and stratum distribution map

1—secondary mast; 2—main mast; 3—air supply hose; 4—power head; 5—gas storage tank; 6—drill rod; 7—air compressor; 8—sleeve; 9—sleeve support frame; 10—Deep hole impactor; 11—Jiangshui; 12—Platform frame; 13—Drill base frame; 14—Power; 15—Hydraulic motor and console; 16—Slant support handle

2. Construction process:

1) The upper part adopts the casing isolation retaining wall method

The project uses a GC-100 (or W-150) down-the-hole impactor flange to connect to a 127 mm core tube (Figure 3). The air supply causes the downhole impactor to be impacted and the rig is pressurized, and the core tube is directly driven into the gravel layer until it is not advanced; then the rig is turned on and the impact is turned to the core tube to sit firmly in the bedrock of the weathering layer. At this time, the lower part of the core tube no longer returns to the wind, and there is no air bubble in the river water; if the large gravel stone is encountered, the core tube does not reach the bedrock, then the downhole impactor and the core tube flange are unloaded, and then the downhole impactor is Change to a small class of WC-85 type down-the-hole impactor (Fig. 4), drill in the 127 mm core tube, remove the core and advance ahead, break the large gravel and continue to drill the core until The core tube sits on a stable bedrock. Leave the 127 mm core tube in the hole as a sleeve.


Figure 3 Schematic diagram of the gravel drilling tool

1-73 mm drill pipe; 2-reducing joint; 3-W-150 impactor; 4--spline joint;

5—Flange; 6-127 mm core tube; 7-130mm tube boots


Figure 4 Schematic diagram of bedrock drilling tool

1-73 mm drill pipe; 2-reducing joint; 3-WC-85 impactor;

4—spline joint; 5-100 ball bit drill

(2) Bedrock formation drilling

After the casing is fixed, the WC-85 type DTH impactor is equipped with a 110 mm cylindrical tooth drill to drill in the casing to remove the gravel layer and core. The operation adopts slow rotation, light pressure, diligent lifting and large air volume regulation. In order to avoid excessive cuttings, it is caught in the annular gap between the impactor and the casing, causing a stuck drill accident, or driving the casing to pull up or destroy the casing stability.

When the impactor exceeds the bottom end of the casing by 0.3 m, rapid drilling is used. In the case of stratum fracture, fissure, and stomatal development, the flow rate of the upper return is reduced due to the gas and leakage in the stratum. In order to eliminate the debris, the drill should be vigorously blown to ensure the normal drilling.

The construction shows that the pneumatic down-the-hole impactor is drilled into the gravel layer, hard fractured and cracked development stratum. It not only has high drilling efficiency, but also has long bit life. It uses compressed gas as the flushing medium to solve the problem due to low temperature (-39 °C). The problem of road freezing. The project not only summed up a set of underwater drilling experience. And achieved good economic and social benefits.

3. DTH hole impactor synchronous heel drilling method

That is, the downhole impactor is drilled while the casing is inserted into the hole along with the drill bit. The follow-up casing has the functions of stabilizing the hole wall and protecting the hole, and the three processes of drilling, slag discharge and wall protection are simultaneously performed, so that the drilling work can be smoothly performed. This is a kind of technological measure that takes advantage of the high efficiency of the downhole impactor crushing rock and tries to overcome the poor wall protection performance. Currently used for production are:

Sweden's "ODEX" eccentric reaming method, Germany's "Saturn" and "Neptune" double-rotation drilling method, the United States Ingersoll Rand's centroid reaming method. The author developed two single-eccentric and double-eccentric eccentric reaming drills for the Maanshan project in Hong Kong. The structure is shown in Figure 5 and Figure 6.


Figure 5 Single eccentric reaming drill

1—cutting the bottom lip of the cutting tooth; 2—pin; 3—stop; 4—bit body


Figure 6 Double eccentric drill

1—cutting plate; 2—pin shaft; 3—positioning pin; 4—stop ring; 5—bit body

KD-560/520 and KS-560/520 two kinds of eccentric reaming drills have simple structure and convenient processing. The production test shows that the eccentric reaming drill has reliable stretching and shrinking action and long service life.

Through-drilling impactor for complex formation drilling

In order to achieve continuous reverse core (sample) drilling, the author developed two types of through-hole impactor GQ-200/62 and GQ-100/44 in the late 1980s. Reverse circulation continuous core (sample) through-type down-hole impactor drilling is a set of down-hole impactor drilling, reverse circulation drilling, continuous coring (sample) drilling in one, comprehensive application of a variety of advanced drilling technology .

The principle of continuous core (sample) drilling of the reversed-hole impactor is shown in Fig. 

7. Compressed air enters the double-channel faucet, reaches the down-hole impactor through the active drill pipe and the double-walled drill pipe, drives the piston, generates high-frequency impact, and the drill is still driven by the drill, so that the drill bit has both impact and rotation to break the rock. The reverse circulation special cylindrical tooth bit crushes the rock at the bottom of the hole to produce a cylindrical core. After the impactor works, the compressed air is sprayed from the bottom of the drill bit, the cuttings are blown at the bottom of the hole and the drill bit is cooled, and then the flow of the drill lip is introduced into the center hole of the drill bit, and then through the center channel of the double wall drill pipe. Carry the core (sample), transport it to the center hole of the two-channel gas faucet, and enter the core (sample) collector through the gooseneck tube and the corrugated tube.


Fig.7 Schematic diagram of the continuous core (sample) drilling of the through-type pneumatic down-the-hole impactor

1—line heart (sample) tube; 2—double channel gas faucet; 3—goose neck bend tube; 4—air intake hose; 5—double wall drill pipe; 6—check valve; 7—heart tube; Cylinder; 9-piston; 10-bushing; 11-reverse circulation special drill

The characteristics of the drilling method are mainly: the high efficiency of the rock using the air latent hole impactor, the high return flow rate of the flushing liquid in the reverse circulation system, and the strong ability to carry the core (sample), and the double wall drill pipe not only transmits the compressed air but also has the function of retaining the wall. The outer ring gap has no flushing liquid medium and the discharge of the effluent, and the disturbance to the hole wall is slight, effectively unifying the two original divisions of the rock breaking and extracting the core (sample) into a continuous operation system, becoming a side drill A new drilling process that is carried out simultaneously with the three types of operating procedures, such as entering and sourcing the core (sample) and washing the well.

The results have formed a series of products, in which the GQ-100/44 type down-the-hole impactor has been tested by the mining bureaus of Henan, Ningxia, Anhui and other places, with a total footage of more than 2,000 m and a maximum hole depth of 259.2 m, in a hard fractured stratum. The average average aging is 4.08 m/h, the monthly efficiency is 1065.17 m, and the maximum drilling interval is 86 m. Compared with the diamond drilling in the same mining area, the drilling efficiency is increased by 3 times, and the monthly efficiency is increased by 5 to 8 times. The cost is reduced by 5 to 8 times.

GQ-200/62 type through-hole impactor is mainly used for hydrological well drilling. It is used by the Henan First Hydrographic Team and has an aging effect of 9.16 m in the drillable grade 7 limestone formation (only one air volume is 10). M3/min air compressor), the reverse cycle core rate is 100%, and a cylindrical tooth drill bit 104.5 m is basically wear-free. The drilling tool and drilling method have the following advantages:

1) The through-hole type impactor and the hollow section of the double-walled drill pipe are used as the reverse circulation channel. Due to the small section and the constant diameter, the upper return wind speed can be increased under the condition of limited air supply, and the slagging capacity can be improved to avoid the rock at the bottom of the hole. Repeated crushing to improve drilling efficiency and depth of use of the downhole impactor;

2) The slagging effect is not limited by the borehole diameter, so it can be holed at one diameter, which changes the process of “small diameter and large diameter expansion” in the past, thus simplifying the construction procedure, improving efficiency and shortening the construction period;

3) It can be combined with gas lift reverse circulation drilling to realize the rockburst and mud reverse circulation retaining wall of the downhole impactor, which can solve the problem of instability of sand gravel, broken collapsed stratum and covered stratum drilling hole wall;

4) In the fractured fracture development stratum and the lost stratum, the reverse circulation is stable and reliable, not only can protect the pore wall, the rock core take rate is 100%, and the block size is large, the quality is good, and the layer is correct and timely; during the drilling process There will be no leakage in the hole or no slag discharge in the orifice, and no accident will occur in the hole;

5) The drilling process is the well washing process, which does not block the aquifer and increases the water output of the well;

6) There is no returning material in the middle hole, no matter whether it is dry hole section, wet layer or underwater drilling, the cutting dust is discharged by the slag discharge pipe, no dust pollution and blowout hazard are generated, and the working conditions are improved.

Large diameter down-the-hole impactor drilled in boulder and boulders

Randomly distributed in the soft layer such as silty clay, flowing sand and other unweathered large lone stone, boulders, due to the hard and compact lithology, smooth surface, drillability of 9 to 10, a single diameter of 0.5 m ~ 7 m Etc. is a major problem in the construction of large diameter engineering wells. The main problem is that the hard rock drilling efficiency is extremely low, the rock surface is smooth, the drill bit has a poor positive guiding effect, and the hole inclination is seriously out of tolerance. The friction between the hole and the rock causes the drill pipe to break and the drill bit accident (Fig. 8).


Figure 8 The drill bit slides down the rock face

1—drill pipe; 2—cage drill bit; 3—advanced pilot bit

When using a down-the-hole impactor to drill in such a type of formation, in order to obtain the desired effect, sufficient impact energy must be achieved on the rock per unit area of crushing. Therefore, as the diameter of the well increases, the lithology becomes hard and the air consumption must be increased. The matching air compressors have to be increased to save energy and reduce one-time investment.

The author conducted a pilot study on the combined drilling process of the DTH impactor and the blasting method. The drilling process has the following types.

Probe hole pre-explosion method

That is, in the in-situ of the large well, drilling small holes with a small-diameter wind-driven down-hole impactor is not only a supplementary survey, but also the lithology and the distribution of the lone stone and the boulders. The blasthole of the blasting of the charge is directly drilled by the large-diameter down-hole impactor after the large rock block is broken. The drilling process is shown in Figure 9. The pre-explosion method of the exploration hole is pre-explosion in the original rock, the charge amount can be larger, the blasting effect will be better, and the collapse of the hole wall need not be taken care of. In the construction of the first stage cofferdam anti-seepage wall of the Yangtze Three Gorges Water Control Project, a total of 62 holes were pre-exploded, which achieved very satisfactory results.


Figure 9 Schematic diagram of the process of pre-explosion method

1—small borehole; 2—soft stratum; 3—explosive pack; 4—large boulder, boulders; 5—bedrock; 6—large boulder and boulders after frying; 7—large diameter dive hole impact 8 - DTH drill bit; a - pre-drilling hole and charge; b - large boulder and boulders after blasting; c - drilling with large diameter downhole impactor

2. Cylindrical drill bit for ring groove crushing rock + blasting + reverse circulation slagging

The drilling process is as follows: the down-hole impactor is drilled with a cylindrical drill bit to form a core and then an explosive charge is placed from the drill pipe (gravel bomb → fragile fracture core → re-send air to make the down-hole impactor work to be crushed by the top of the cylindrical drill bit Core → reuse pump to reverse circulation chip removal). This is repeatedly drilled into the hole, and the drilling process is shown in Fig. 10.


Figure 10 Impacting into a core along the hole - blasting - slag drilling process diagram

1—drill pipe; 2—FGC-15 type through-drilling hole impactor; 3—cylinder bit top cover hard alloy; 4—gravel bomb; 5—crunch; 6—big boulders

There are many blasting methods, mainly including blasting and concentrating blasting. The following advantages and disadvantages exist in the comparison:

1) Eye blasting method

That is to say, when the drilling and drilling encounters the large lone stone and the boulders, one or more small holes are drilled in the rock block, and the medicine is blasted in the small hole. Compared with the pre-explosion method, the method can save the drilling workload, but drilling the small hole in the large hole must first lower the small casing, which also causes the drilling tool to be skewed, and it is difficult to fix the casing. In practice, small diameter diamond drill bits have been used for drilling. Because the speed cannot be increased, the result is not only low efficiency, short life of the drill bit, high cost, but also the large diameter DTH impactor drill in the hole when the above hole is drilled. Proposed that the auxiliary time was increased.

2) Shaped energy blasting method

The use of holes at one end of the column to increase the effect of local damage is called a concentrating blast. The influence of the energy of the concentrating energy is related to the following factors: 1 explosive performance; 2 material, shape, cone angle, wall thickness of the lower end of the drug column; 3 high; 4 shell and geometry of the drug package. Figure 11 (a) is a cover with no tapered holes at the end of the drug column. The explosive product scatters around the surface of the drug column. The only action on the bottom surface is the explosive drug scattered from the end of the drug column. It is equal to the end face area of the drug column, so only a very shallow pit is blown out at the bottom end. In Fig. 11, the dose of the (b) drug column is smaller than that of the (a) drug package, but since there is a tapered hole at the lower end, a deep pit is blown out at the bottom after the blasting; (c) is the lower end of the drug column in Fig. 11 A tapered conical bushing (also known as a blister) is embedded in the surface of the tapered cavity. This blaster explodes and deepens a deeper pit at the bottom. Experiments show that different bottom cones have different damage effects. Large-diameter engineering wells use submerged hole impactor and concentrated energy blasting combined with compound drilling technology in hard rock, lone stone and boulders, which can improve drilling efficiency and overcome the problems of large lone stone and boulders. The hole is inclined. It is a construction method with simple process, safe and easy operation, energy saving and low cost.

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